Solar cells may be defined as devices to convert light energy into electrical energy by using a photovoltaic effect of generating electrons when light is incident onto a P-N junction diode. The solar cell may be classified into a silicon solar cell, a compound semiconductor solar cell mainly including a group I-III-VI compound or a group III-V compound, a dye-sensitized solar cell, and an organic solar cell according to materials constituting the junction diode.
A solar cell made from CIGS (CuInGaSe), which is one of group I-III-VI Chal-copyrite-based compound semiconductors, represents superior light absorption, higher photoelectric conversion efficiency with a thin thickness, and superior electro-optic stability, so the CIGS solar cell is spotlighted as a substitute for a conventional silicon solar cell.
FIG. 1 shows the structure of a CIGS thin film solar cell according to the related art. In general, the CIGS solar cell can be fabricated by sequentially forming a back electrode layer 20, a light absorbing layer 30, a buffer layer 40 and a front electrode layer 50 on a glass substrate 10. The substrate 10 can be prepared by using various materials, such as soda lime glass, stainless steel and polyimide (PI).
The light absorbing layer 30 is a P type semiconductor layer and mainly includes CuInSe2 or Cu(InxGal-x)Se2, which is obtained by replacing a part of In with Ga. The light absorbing layer can be formed through various processes, such as an evaporation process, a sputtering process, a selenization process or an electroplating process.
The buffer layer 40 is disposed between the light absorbing layer and the front electrode layer, which represent great difference in lattice coefficient and energy bandgap, to form a superior junction therebetween. The buffer layer 40 mainly includes cadmium sulfide prepared through chemical bath deposition (CBD).
The front electrode layer 60 is an N type semiconductor layer and forms a PN junction with respect to the light absorbing layer 30 together with the buffer layer 40. In addition, since the front electrode layer serves as a transparent electrode at a front surface of the solar cell, the front electrode layer mainly includes aluminum-doped zinc oxide (AZO) having the superior light transmittance and electric conductivity.
Different from a bulk solar cell, the CIGS thin film solar cell includes a plurality of unit cells, which are connected with each other in series through patterning processes (TH1 to TH3). The most important patterning process is the TH2 process. A connection wire 70 makes contact with the back electrode 20 at the TH2 pattern, so electric loss may occur and the efficiency of the solar cell may be significantly degraded if the contact fails at the TH2 pattern.
Meanwhile, referring to FIG. 1, a selenium-molybdenum layer 21 can be formed while the light absorbing layer 30 is being formed on the back electrode 20. In detail, the selenium-molybdenum layer 21 may be formed through the reaction between molybdenum (Mo) of the back electrode and selenium of the light absorbing layer 30. However, the selenium-molybdenum layer 21 may increase the contact resistance between the connection wire 70 and the back electrode 20.